[PATCH] sched: resched and cpu_idle rework

Make some changes to the NEED_RESCHED and POLLING_NRFLAG to reduce
confusion, and make their semantics rigid.  Improves efficiency of
resched_task and some cpu_idle routines.

* In resched_task:
- TIF_NEED_RESCHED is only cleared with the task's runqueue lock held,
  and as we hold it during resched_task, then there is no need for an
  atomic test and set there. The only other time this should be set is
  when the task's quantum expires, in the timer interrupt - this is
  protected against because the rq lock is irq-safe.

- If TIF_NEED_RESCHED is set, then we don't need to do anything. It
  won't get unset until the task get's schedule()d off.

- If we are running on the same CPU as the task we resched, then set
  TIF_NEED_RESCHED and no further action is required.

- If we are running on another CPU, and TIF_POLLING_NRFLAG is *not* set
  after TIF_NEED_RESCHED has been set, then we need to send an IPI.

Using these rules, we are able to remove the test and set operation in
resched_task, and make clear the previously vague semantics of
POLLING_NRFLAG.

* In idle routines:
- Enter cpu_idle with preempt disabled. When the need_resched() condition
  becomes true, explicitly call schedule(). This makes things a bit clearer
  (IMO), but haven't updated all architectures yet.

- Many do a test and clear of TIF_NEED_RESCHED for some reason. According
  to the resched_task rules, this isn't needed (and actually breaks the
  assumption that TIF_NEED_RESCHED is only cleared with the runqueue lock
  held). So remove that. Generally one less locked memory op when switching
  to the idle thread.

- Many idle routines clear TIF_POLLING_NRFLAG, and only set it in the inner
  most polling idle loops. The above resched_task semantics allow it to be
  set until before the last time need_resched() is checked before going into
  a halt requiring interrupt wakeup.

  Many idle routines simply never enter such a halt, and so POLLING_NRFLAG
  can be always left set, completely eliminating resched IPIs when rescheduling
  the idle task.

  POLLING_NRFLAG width can be increased, to reduce the chance of resched IPIs.

Signed-off-by: Nick Piggin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Con Kolivas <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Documentation/sched-arch.txt

@@ -0,0 +1,89 @@
+	CPU Scheduler implementation hints for architecture specific code
+
+	Nick Piggin, 2005
+
+Context switch
+==============
+1. Runqueue locking
+By default, the switch_to arch function is called with the runqueue
+locked. This is usually not a problem unless switch_to may need to
+take the runqueue lock. This is usually due to a wake up operation in
+the context switch. See include/asm-ia64/system.h for an example.
+
+To request the scheduler call switch_to with the runqueue unlocked,
+you must `#define __ARCH_WANT_UNLOCKED_CTXSW` in a header file
+(typically the one where switch_to is defined).
+
+Unlocked context switches introduce only a very minor performance
+penalty to the core scheduler implementation in the CONFIG_SMP case.
+
+2. Interrupt status
+By default, the switch_to arch function is called with interrupts
+disabled. Interrupts may be enabled over the call if it is likely to
+introduce a significant interrupt latency by adding the line
+`#define __ARCH_WANT_INTERRUPTS_ON_CTXSW` in the same place as for
+unlocked context switches. This define also implies
+`__ARCH_WANT_UNLOCKED_CTXSW`. See include/asm-arm/system.h for an
+example.
+
+
+CPU idle
+========
+Your cpu_idle routines need to obey the following rules:
+
+1. Preempt should now disabled over idle routines. Should only
+   be enabled to call schedule() then disabled again.
+
+2. need_resched/TIF_NEED_RESCHED is only ever set, and will never
+   be cleared until the running task has called schedule(). Idle
+   threads need only ever query need_resched, and may never set or
+   clear it.
+
+3. When cpu_idle finds (need_resched() == 'true'), it should call
+   schedule(). It should not call schedule() otherwise.
+
+4. The only time interrupts need to be disabled when checking
+   need_resched is if we are about to sleep the processor until
+   the next interrupt (this doesn't provide any protection of
+   need_resched, it prevents losing an interrupt).
+
+	4a. Common problem with this type of sleep appears to be:
+	        local_irq_disable();
+	        if (!need_resched()) {
+	                local_irq_enable();
+	                *** resched interrupt arrives here ***
+	                __asm__("sleep until next interrupt");
+	        }
+
+5. TIF_POLLING_NRFLAG can be set by idle routines that do not
+   need an interrupt to wake them up when need_resched goes high.
+   In other words, they must be periodically polling need_resched,
+   although it may be reasonable to do some background work or enter
+   a low CPU priority.
+
+   	5a. If TIF_POLLING_NRFLAG is set, and we do decide to enter
+	    an interrupt sleep, it needs to be cleared then a memory
+	    barrier issued (followed by a test of need_resched with
+	    interrupts disabled, as explained in 3).
+
+arch/i386/kernel/process.c has examples of both polling and
+sleeping idle functions.
+
+
+Possible arch/ problems
+=======================
+
+Possible arch problems I found (and either tried to fix or didn't):
+
+h8300 - Is such sleeping racy vs interrupts? (See #4a).
+        The H8/300 manual I found indicates yes, however disabling IRQs
+        over the sleep mean only NMIs can wake it up, so can't fix easily
+        without doing spin waiting.
+
+ia64 - is safe_halt call racy vs interrupts? (does it sleep?) (See #4a)
+
+sh64 - Is sleeping racy vs interrupts? (See #4a)
+
+sparc - IRQs on at this point(?), change local_irq_save to _disable.
+      - TODO: needs secondary CPUs to disable preempt (See #1)
+

arch/alpha/kernel/process.c

@@ -43,21 +43,17 @@
 #include "proto.h"
 #include "pci_impl.h"
 
-void default_idle(void)
-{
-	barrier();
-}
-
 void
 cpu_idle(void)
 {
+	set_thread_flag(TIF_POLLING_NRFLAG);
+
 	while (1) {
-		void (*idle)(void) = default_idle;
 		/* FIXME -- EV6 and LCA45 know how to power down
 		   the CPU.  */
 
 		while (!need_resched())
-			idle();
+			cpu_relax();
 		schedule();
 	}
 }

arch/arm/kernel/process.c

@@ -86,12 +86,16 @@ EXPORT_SYMBOL(pm_power_off);
  */
 void default_idle(void)
 {
-	local_irq_disable();
-	if (!need_resched() && !hlt_counter) {
-		timer_dyn_reprogram();
-		arch_idle();
+	if (hlt_counter)
+		cpu_relax();
+	else {
+		local_irq_disable();
+		if (!need_resched()) {
+			timer_dyn_reprogram();
+			arch_idle();
+		}
+		local_irq_enable();
 	}
-	local_irq_enable();
 }
 
 /*

arch/i386/kernel/apm.c

@@ -769,8 +769,26 @@ static int set_system_power_state(u_short state)
 static int apm_do_idle(void)
 {
 	u32	eax;
+	u8	ret = 0;
+	int	idled = 0;
+	int	polling;
+
+	polling = test_thread_flag(TIF_POLLING_NRFLAG);
+	if (polling) {
+		clear_thread_flag(TIF_POLLING_NRFLAG);
+		smp_mb__after_clear_bit();
+	}
+	if (!need_resched()) {
+		idled = 1;
+		ret = apm_bios_call_simple(APM_FUNC_IDLE, 0, 0, &eax);
+	}
+	if (polling)
+		set_thread_flag(TIF_POLLING_NRFLAG);
+
+	if (!idled)
+		return 0;
 
-	if (apm_bios_call_simple(APM_FUNC_IDLE, 0, 0, &eax)) {
+	if (ret) {
 		static unsigned long t;
 
 		/* This always fails on some SMP boards running UP kernels.

arch/i386/kernel/process.c

@@ -99,14 +99,22 @@ EXPORT_SYMBOL(enable_hlt);
  */
 void default_idle(void)
 {
+	local_irq_enable();
+
 	if (!hlt_counter && boot_cpu_data.hlt_works_ok) {
-		local_irq_disable();
-		if (!need_resched())
-			safe_halt();
-		else
-			local_irq_enable();
+		clear_thread_flag(TIF_POLLING_NRFLAG);
+		smp_mb__after_clear_bit();
+		while (!need_resched()) {
+			local_irq_disable();
+			if (!need_resched())
+				safe_halt();
+			else
+				local_irq_enable();
+		}
+		set_thread_flag(TIF_POLLING_NRFLAG);
 	} else {
-		cpu_relax();
+		while (!need_resched())
+			cpu_relax();
 	}
 }
 #ifdef CONFIG_APM_MODULE
@@ -120,29 +128,14 @@ EXPORT_SYMBOL(default_idle);
  */
 static void poll_idle (void)
 {
-	int oldval;
-
 	local_irq_enable();
 
-	/*
-	 * Deal with another CPU just having chosen a thread to
-	 * run here:
-	 */
-	oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-
-	if (!oldval) {
-		set_thread_flag(TIF_POLLING_NRFLAG);
-		asm volatile(
-			"2:"
-			"testl %0, %1;"
-			"rep; nop;"
-			"je 2b;"
-			: : "i"(_TIF_NEED_RESCHED), "m" (current_thread_info()->flags));
-
-		clear_thread_flag(TIF_POLLING_NRFLAG);
-	} else {
-		set_need_resched();
-	}
+	asm volatile(
+		"2:"
+		"testl %0, %1;"
+		"rep; nop;"
+		"je 2b;"
+		: : "i"(_TIF_NEED_RESCHED), "m" (current_thread_info()->flags));
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -181,6 +174,8 @@ void cpu_idle(void)
 {
 	int cpu = smp_processor_id();
 
+	set_thread_flag(TIF_POLLING_NRFLAG);
+
 	/* endless idle loop with no priority at all */
 	while (1) {
 		while (!need_resched()) {
@@ -246,15 +241,12 @@ static void mwait_idle(void)
 {
 	local_irq_enable();
 
-	if (!need_resched()) {
-		set_thread_flag(TIF_POLLING_NRFLAG);
-		do {
-			__monitor((void *)&current_thread_info()->flags, 0, 0);
-			if (need_resched())
-				break;
-			__mwait(0, 0);
-		} while (!need_resched());
-		clear_thread_flag(TIF_POLLING_NRFLAG);
+	while (!need_resched()) {
+		__monitor((void *)&current_thread_info()->flags, 0, 0);
+		smp_mb();
+		if (need_resched())
+			break;
+		__mwait(0, 0);
 	}
 }
 

arch/ia64/kernel/process.c

@@ -197,11 +197,15 @@ void
 default_idle (void)
 {
 	local_irq_enable();
-	while (!need_resched())
-		if (can_do_pal_halt)
-			safe_halt();
-		else
+	while (!need_resched()) {
+		if (can_do_pal_halt) {
+			local_irq_disable();
+			if (!need_resched())
+				safe_halt();
+			local_irq_enable();
+		} else
 			cpu_relax();
+	}
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
@@ -263,16 +267,16 @@ void __attribute__((noreturn))
 cpu_idle (void)
 {
 	void (*mark_idle)(int) = ia64_mark_idle;
+  	int cpu = smp_processor_id();
+	set_thread_flag(TIF_POLLING_NRFLAG);
 
 	/* endless idle loop with no priority at all */
 	while (1) {
+		if (!need_resched()) {
+			void (*idle)(void);
 #ifdef CONFIG_SMP
-		if (!need_resched())
 			min_xtp();
 #endif
-		while (!need_resched()) {
-			void (*idle)(void);
-
 			if (__get_cpu_var(cpu_idle_state))
 				__get_cpu_var(cpu_idle_state) = 0;
 
@@ -284,19 +288,17 @@ cpu_idle (void)
 			if (!idle)
 				idle = default_idle;
 			(*idle)();
-		}
-
-		if (mark_idle)
-			(*mark_idle)(0);
-
+			if (mark_idle)
+				(*mark_idle)(0);
 #ifdef CONFIG_SMP
-		normal_xtp();
+			normal_xtp();
 #endif
+		}
 		preempt_enable_no_resched();
 		schedule();
 		preempt_disable();
 		check_pgt_cache();
-		if (cpu_is_offline(smp_processor_id()))
+		if (cpu_is_offline(cpu))
 			play_dead();
 	}
 }

arch/parisc/kernel/process.c

@@ -88,6 +88,8 @@ void default_idle(void)
  */
 void cpu_idle(void)
 {
+	set_thread_flag(TIF_POLLING_NRFLAG);
+
 	/* endless idle loop with no priority at all */
 	while (1) {
 		while (!need_resched())

arch/powerpc/platforms/iseries/setup.c

@@ -703,13 +703,10 @@ static void iseries_shared_idle(void)
 static void iseries_dedicated_idle(void)
 {
 	long oldval;
+	set_thread_flag(TIF_POLLING_NRFLAG);
 
 	while (1) {
-		oldval = test_and_clear_thread_flag(TIF_NEED_RESCHED);
-
-		if (!oldval) {
-			set_thread_flag(TIF_POLLING_NRFLAG);
-
+		if (!need_resched()) {
 			while (!need_resched()) {
 				ppc64_runlatch_off();
 				HMT_low();
@@ -722,9 +719,6 @@ static void iseries_dedicated_idle(void)
 			}
 
 			HMT_medium();
-			clear_thread_flag(TIF_POLLING_NRFLAG);
-		} else {
-			set_need_resched();
 		}
 
 		ppc64_runlatch_on();